kernel: separate headers into kernel and user headers

all kernel headers have been moved from include/mango to include/kernel
and include definitions that are only relevant to kernel-space.

any definitions that are relevant to both kernel- and user-space
(i.e. type definitions, syscall IDs) have been moved to
include/mango within libmango.

arch/user/stdcon.c

@@ -1,10 +1,10 @@
-#include <mango/libc/string.h>
-#include <mango/libc/ctype.h>
+#include <kernel/libc/string.h>
+#include <kernel/libc/ctype.h>
 #include <stdint.h>
 #include <stdio.h>
-#include <mango/console.h>
-#include <mango/vm.h>
-#include <mango/printk.h>
+#include <kernel/console.h>
+#include <kernel/vm.h>
+#include <kernel/printk.h>
 
 static void stdcon_write(struct console *con, const char *s, unsigned int len)
 {